The present invention relates generally to the field of control systems for pneumatic actuators. More specifically, the present invention relates to a servo control system that provides precise and repeatable control of a pneumatic actuator.
Mechanical actuators are used in a variety of industrial applications to move machine elements from one position to another. There are three different ways to power the movement of an actuator, electrically, hydraulically or pneumatically. Electrically powered actuators are used in situations requiring precise control and repeatability. An electrically powered actuator such as a screw drive or belt drive system powered by a rotary servo motor system has the ability to move at different speeds and to stop at any location along the entire length of the stroke of the actuator. Unfortunately, electrically powered actuators are prohibitively expensive for applications involving moving large loads or moving loads at rapid speeds. Hydraulically and pneumatically powered actuators, on the other hand, use a fluid (oil for hydraulics vs. air or gas for pneumatics) to provide a substantial force to a piston that moves inside a chamber and is connected to the actuator. Consequently, hydraulic and pneumatic actuators can move large loads and can move those loads at rapid speeds if desired. Hydraulic and pneumatic actuators also tend to be more durable than electrical actuators.
While hydraulic actuators are well suited for many applications, their use is limited to those environments where oil can be used as part of the machine. For many applications, it is often not practical to utilize oil as the fluid to power an actuator. Pneumatic actuators also tend to be less expensive than hydraulic actuators or electrical actuators. The problem is that pneumatic actuators are much more difficult to precisely control than electrical actuators or even hydraulic actuators. Consequently, most pneumatic actuators are designed to position the actuator at only two stop positions, one at each end of the stroke of the actuator where the end of the chamber, a stopper or the like serves to physically stop the travel of the piston, thereby positioning the actuator at one of these two stop positions. This kind of two stop pneumatic actuator is controlled simply by supplying pressurized air to one side of the piston until the piston reaches the end of the stroke.
To control a pneumatic actuator to stop at positions other than the ends of the stroke of the actuator, the most common technique is to supply air at different pressures to both sides of the piston. Initially, this differential pressure will start the piston moving in a direction from the side with the higher pressure to the side with the lower pressure. Once the piston is moving, this differential pressure is reversed to cause the piston to stop moving. Ideally, the differential pressures can be applied to cause the piston to start and stop exactly at any desired location along the stroke of the actuator. In reality, adjusting the differential pressure to achieve the delicate balance required to precisely control the stop positions of the actuator is quite difficult due at least in part to the compressibility of the air or gas that is used as the fluid to power and control the actuator. These problems are compounded in situations involving changing loads, long stroke lengths or vertically-oriented stroke directions, or in situations where the pressure of the air or gas used to power the actuator is not tightly controlled.
The most common way of adjusting the differential pressure for this kind of pneumatic actuator is by controlling a variable valve or pair of variable valves, which are sometimes referred to as proportional valves or servo valves. Examples of control systems developed for differential pneumatic actuators that use proportional valves to control the differential pressure are described in U.S. Pat. Nos. 4,481,451, 4,666,374, 4,790,233, 4,819,543, 5,154,207 and 6,003,428 and German Patent DE 3313 623 A1. Other variations on controlling a differential pneumatic actuator are described in U.S. Pat. No. 4,878,417 which varies the proportional flow of the fluid in response to measurements from an accelerometer and U.S. Pat. No. 5,424,941 which uses a control system that converts differential pressure into a differential mass flow of the air that moves the piston in an attempt to minimize the problems caused by the compressibility of air.
An alternative technique for controlling the differential pressure of a pneumatic actuator is to use pulse width modulation (e.g., different widths of control pulses) to control the supply of pressurized air to both sides of the piston. Instead of turning a servo valve part way on to control the rate that air flows through the valve, pulse width modulation controls the rate by quickly turning the valve on and then off such that the average time the valve is on is equivalent to the proportional setting of a valve turned part way on for the same period of time. Examples of this pulse width modulation technique are described in U.S. Pat. Nos. 4,628,499, 4,763,560 and 4,907,493. U.S. Pat. No. 4,741,247 describes a very slow version of a pulse width modulation scheme where a series of step volumes of air are introduced into the chamber one at a time in order to move the piston a distance equal to the step volume.
Various attempts have been made over the years to address the problems caused by using air as the fluid to power a pnuematic actuator. One approach has been to use some form of a brake to assist in stopping the piston or the actuator. German patent DE 2,327,387 describes an early use of an electromagnetic friction brake to stop a pneumatic actuator. This patent uses a conventional proportional servo valve to control the differential pressure. Once the actuator passes by a predetermined starting point for braking, the electromagnetic friction brake is applied intermittently to slow the piston down until it is moving at a much slower speed, at which time the brake is applied continuously to completely stop the actuator.
One of the problems with using a brake; however, is that the brake surface will wear down with repeated use and this results in variability in how accurately the system operates over time. U.S. Pat. No. 4,106,390 describes a pneumatic linear actuator where a pneumatic mechanical brake is activated in a braking cylinder separate from but connected to the piston to prevent wear directly on the piston. The pneumatic mechanical brake is only applied to stop the actuator after a three-stage series of air braking decelerations are performed by operating solenoids in response to output signals generated by a sequence generator.
Other types of brakes have also been used as part of a control system for a pneumatic actuator. U.S. Pat. No. 4,932,311 describes a pneumatic actuator having a magnetic rotary brake coupled to the piston by a ball screw shaft. A two-stage braking scheme based on a target braking speed is used to control the stopping locations of the piston. Once the piston passes the location where braking has been programmed to start, either or both an air braking arrangement and the magnetic brake may be applied at different periods along a braking process in either an intermittent or a continuous mode to keep the speed of the piston on target with a calculated braking speed. By attempting to control the speed of the actuator to match the calculated braking speed, the patent seeks to regulate the pneumatic actuator in a way that can tolerate and compensate for changes in the system, including changes in the brake.
A recent example of a controllable pneumatic actuator that uses a rotary proportional magnetic brake is described in PCT Publ. No. WO 00/53936. Both a simple control system and a sophisticated control system are described. In the simple control system, differential pressure is applied though a three-position solenoid valve to drive the pneumatic actuator until it is within a defined distance (Ax) on either side of the desired stopping position that is referred to as a tolerance band. Once inside this tolerance band, the three-position valve is set to a neutral position where no pressurized air is applied to either side of the piston and the proportional magnetic brake is used to stop the actuator. If the load cannot be stopped within the tolerance band it will overshoot the desired stopping position and a reverse thrust must be applied to move the load back into the tolerance band. The problems with the simple control system are that with large loads and speeds where the kinetic energy is high and with system friction present, it becomes difficult to deal with the compressed air required to overcome the system friction and start the load moving and then stop the load moving in time to keep it within the tolerance band. This latter problem is a very undesirable problem known as hunting where the actuator goes back and forth about the desired stopping position or in this case, the defined tolerance band, before finally being stopped.
The sophisticated control system described in this PCT application attempts to solve the problem of hunting in those cases where the kinetic energy of the load being moved is greater than the braking force of the magnetic brake. In this case, the control system calculates a shut down point that will be prior to the desired stopping point based upon the kinetic energy of the system and the available braking force. The shut down point represents the exact time that the system will shut down the differential pressure applied to the actuator and start the magnetic brake. The control system allows a user to input a desired velocity profile for the actuator and uses a low level brake signal to maintain that velocity at the programmed desired velocity up to the shut down point. While the calculation of a shut down point based on kinetic energy may be helpful in reducing the hunting problem of the simple control system, it does not address the real world problems of precision and repeatability in accurately positioning the actuator at a desired stopping position. To be effective, the calculation of the shut down point requires precise knowledge about the mass of the load and the variability of the braking force. Even if precise knowledge of these values can be programmed into the control system, this sophisticated control system is unable to accommodate changes in loads during an operation or changes in the remaining parts of the system due to wear or variance.
In spite of these various attempts, the control and use of pneumatic actuators has been unable to match the precision and repeatability of electrically powered actuators. The goal of being able to program a pneumatic actuator in the same way that an electrical actuator is programmed is well known, but has yet to be achieved. Ideally, a pneumatic actuator could be controlled merely by setting a desired motion profile and/or stopping position and then relying on a servo control system to use positional feedback to guarantee that those results will be achieved. If this were possible, pneumatic actuators could be used in a variety of situations that up to now have been the exclusive domain of electrical actuators. Unfortunately, the number of variables that must be controlled in a pneumatic system, and especially the variabilities caused by the compressibility of air, have frustrated the many attempts to realize this goal.
Because of the complexities and variables involved in operating a pneumatic actuator, most existing control systems are highly individualized to the particular pneumatic actuator and typically require the operator to program the actuator by programming control values for the system in terms of encoder values, pulses, sequences or other units that have complicated relationships to a desired position or velocity. For those control systems which utilize servo systems, the operator must program a very large number of gains in an attempt to fine-tune the control system for a given application. While it may be possible to fine-tune the programming of a given pneumatic actuator with this very large number of gains or adjustments to perform adequately under the known conditions for that given application, even the existing control systems for pneumatic actuators that use servo systems are unable to compensate for situations involving changing loads, long stroke lengths or vertically oriented stroke directions. For this reason, existing control systems for pneumatic actuators limit their performance claims to certain defined conditions, such as limits on the ability to change loads, and always express tolerances in terms of percentages of the stroke length as a way to hide repeatability errors associated with the variability of larger volumes of air that are involved in longer stroke lengths.
It would be desirable to provide an easily programmable servo control system for a pneumatic actuator that could effectively control the operation of the pneumatic actuator to match the precision and repeatability of electrical actuators, even under changing loads, long stroke lengths or vertically oriented stroke directions and that did not require tightly controlled air supplies and expensive servo valves to accomplish this precise servo control.
The present invention is a precision servo control system for a pneumatic actuator that has a piston positionable over a stroke of the pneumatic actuator using a supply of pressurized gas. A brake and a sensor system are operably connected to the actuator and to the servo control system. The servo control system operates to initiate the forward thrust from the pressurized gas to move the piston along the stroke. When the piston reaches a deceleration point along the stroke as preferably determined by a programmable motion profile, the servo control system initiates a reverse thrust from the pressurized gas while maintaining the forward thrust and simultaneously begins to selectively apply the brake to stop the piston within a predetermined tolerance of a desired stopping position. The precision servo control system is easily programmable in a manner similar to that of a control system for electrical actuators with respect to both the motion profile and the limited number of gains of the control system. The precision servo control system can maintain the predetermined tolerance even under changing loads, long stroke lengths or vertically oriented stroke directions. The precision control servo system utilizes simple two position valves, instead of complex and expensive servo valves, to regulate the pressurized gas. The precision servo control system achieves positional repeatability to predetermined tolerance that is a fixed value, regardless of the stroke length.
Preferably, the precision servo control system allows a user to input a desired motion profile for the actuator and uses a low level brake signal to limit the acceleration to the desired acceleration and velocity of the programmed motion profile until the piston reaches the deceleration-point. Preferably, the brake is a proportional magnetic brake that produces increasing braking torque with increasing current applied to the brake. At the deceleration point, the precision servo control system continues to apply gas pressure to the forward side of the piston and also applies an equal and opposite gas pressure to the backward side of the piston. Because these two pressures are equivalent and opposite, they operate to effectively cancel out any contribution of the pressurized gas in the system after the deceleration point. The active application of the same pressurized gas to both the forward and reverse sides of the piston guarantees that there will be no complicated interaction of a reduction of pressures during the deceleration period.
In addition, the precision servo control system also monitors for a predetermined condition after the deceleration point at which the reverse thrust of pressurized gas applied to the backward side of the piston will turned off. The predetermined condition represents the point at which the precision servo control system first determines that either a defined percentage of a distance from the deceleration point to the desired stopping position has been reached or that there has been a defined percentage reduction in the velocity of the piston. The predetermined condition is selected such that the momentum of the load carried by the piston will be sufficiently small enough that it can be accurately controlled by the brake. Removing the reverse thrust at this point insures that the actuator will have the necessary forward momentum to arrive at the desired position without any jerking or stopping of the actuator. In the event that the momentum of the load carried by the piston does cause an overshoot, the precision servo control system locks the brake, then removes the forward thrust and applies the reverse thrust, after which the brake is slowly released to return the piston to the desired stopping position.
Unlike the existing control systems that rely purely on positioning the piston by adjusting the pressure of the column of air on each side of the piston through use of servo control valves or PWM techniques, the precision servo control system of the present invention can use relatively inexpensive directional valves. More importantly, there is no need for the pressurized gas supply to be closely regulated. This means that the precision servo control system of the present invention can tolerate contaminated gas lines and other changes over time in the pressurized gas supply system because the same pressurized gas is supplied to both sides of the piston during the deceleration period. Because of this equal and opposite application of pressure, the characteristics of the gas supply on each side of the piston cancel each other out. As a result, the servo precision control system of the present invention does not need to create complicated higher orders control functions to compensate for the very complicated characteristics of a compressible and changeable gas supply.
In a preferred embodiment, the precision servo control system is programmed utilizing the standard control parameters of a servo-system including: (1) proportional gain, KP; (2) integral gain, KI; and (3) derivative gain, KV. The proportional gain, KP, is the position error gain that determines how sensitively the control system will respond to the position error (difference in command and actual position). The integral gain, KI, is the position error integral gain that determines how the control system responds to accumulated position error while the piston is approaching the target position. The derivative gain, KV, is the speed error gain that determines how effectively the controller/drive 100 responds to the speed error while the piston is in motion. All three of these control parameters are standard control loop gains for a servo system for an electrically powered actuator. In addition to these standard control loop gains, the precision servo control system preferably uses a deceleration compensation path that enters into effect after the braking point. The deceleration compensation path uses a control parameter beyond the standard control loop gain parameters of a servo system for an electrically powered actuator. This control parameter is a deceleration current constant gain, KT that is used to set the minimum brake current while decelerating the load carried by the actuator. The KT gain is used to adjust for position overshoot, position undershoot, and deceleration profile linearity when the actuator approaches the target position. Unlike the complex and higher order control schemes that have been attempted to accommodate for all of the variables in a pneumatic actuator system, the present invention is capable of accurately and repeatably controlling the positioning of a pneumatic actuator with just these four gain parameters KP, KI, KV, and KT.